2. Peel eggs and slice each in half lengthwise with paring knife. Remove yolks to small bowl. Arrange whites on serving platter, discarding two worst-looking halves. Mash yolks with fork until no large lumps remain. Add tuna, capers, 2 teaspoons chives, vinegar, mustard, mayonnaise, and salt and pepper to taste; mix with rubber spatula, mashing mixture against side of bowl until smooth.